mod_cluster request 500s when using a RewriteRule to the balancer with a ProxyPassReverse

Issue

• We use a proxy RewriteRule to the mod_cluster balancer. We added a ProxyPassReverse to this config as well:

<VirtualHost *:80>
    RewriteEngine On
    RewriteRule ^/foo/(.*)$ balancer://mycluster/foo/$1 [P,L]
    ProxyPassReverse /foo/ balancer://mycluster/foo/
</VirtualHost>

• With the ProxyPassReverse in place, requests served through this always 500 with the following log output:

[Thu Aug 08 16:38:50 2013] [debug] mod_proxy_cluster.c(3448): proxy: CLUSTER no balancer for balancer://mycluster/foo/hi.jsp
[Thu Aug 08 16:38:50 2013] [debug] proxy_util.c(1525): [client 127.0.0.1] proxy: *: found reverse proxy worker for balancer://mycluster/foo/hi.jsp
[Thu Aug 08 16:38:50 2013] [debug] mod_proxy.c(1015): Running scheme balancer handler (attempt 0)
[Thu Aug 08 16:38:50 2013] [debug] mod_proxy_http.c(1955): proxy: HTTP: declining URL balancer://mycluster/foo/hi.jsp
[Thu Aug 08 16:38:50 2013] [debug] mod_proxy_ajp.c(664): proxy: AJP: declining URL balancer://mycluster/foo/hi.jsp
[Thu Aug 08 16:38:50 2013] [warn] proxy: No protocol handler was valid for the URL /foo/hi.jsp. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.